misc/ios: ensure deferred cleanup functions are run even on error

log.Fatal exits the process and doesn't allow deferred functions
to run. Extract log.Fatal calls to main where all deferred functions
have completed.

For the iOS builder.

Change-Id: Id1ef9955bed19944a819d6137a611d6ecbe624a6
Reviewed-on: https://go-review.googlesource.com/110955


Run-TryBot: Elias Naur <elias.naur@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>